The present invention relates generally to microstructures. The present invention relates more particularly to a metallic microstructure spring and method for making the same, wherein the metallic microstructure spring is suitable for use as a spring terminus for interconnecting electronic devices such as printed wiring boards and integrated circuits.
Methods for attaching integrated circuits and the like to printed wiring boards (PWBs) are well-known. Such methods enable the fabrication of various electronic subassemblies, such as motherboards and daughterboards for personal computers.
Contemporary methods for attaching integrated circuits to printed wiring boards involve the use of various integrated circuit packaging technologies such as dual in-line package (DIP), plastic lead chip carrier (PLCC), ceramic pin grid array (CPGA), plastic quad flat pack (PQFP), quad flat pack (QFP), tape carrier package (TCP), ball grid array (BGA), thin small outline package gull-wing (TSOP), small outline package J-lead (SOJ), shrink small outline package gull-wing (SSOP) and plastic small outline package (PSOP).
According to DIP packaging technology, the two parallel rows of leads extending from the integrated circuit package pass through holes formed in the printed wiring board and are soldered into the holes. Optionally, a socket may be utilized.
Integrated circuits packaged according to PLCC and CPGA technologies typically require the use of a socket.
PQFP, QFP, TCP, BGA, TSOP, SOJ, SSOP and PSOP are examples of surface mount technology, wherein the packaged integrated circuit is attached directly to a printed wiring board, typically by such techniques as re-flow soldering and/or thermal compression.
For example, BGAs comprise a plurality of electrical contacts formed so as to define a 2-dimensional array upon the bottom surface of an integrated circuit package. Each electrical contact of the BGA comprises a small ball of solder which generally facilitates permanent interconnection of the integrated circuit to a complimentary array of flat electrical contact pads formed upon a printed wiring board. The small solder balls melt during reflow soldering to effect such permanent connection of the integrated circuit to the printed wiring board.
As the number of transistors formed upon a single integrated circuit increases, the attachment of the integrated circuit to a printed wiring board or the like becomes more difficult. This is because integrated circuits having more transistors are more complex and thus generally required more communications pathways to other circuitry. It is expected that the number of transistors formed upon a single integrated circuit will increase from its present number of approximately 80 million to approximately 100 million by the year 2000.
BGAs support high pin counts, so as to facilitate the use of integrated circuits having a large number of transistors formed thereon. By taking advantage of the comparatively large surface area on the bottom of an integrated circuit package, ball grid arrays provide for a comparatively large number of electrical interconnections between the integrated circuit and a printed wiring board.
One problem, which is typically associated with the attachment of integrated circuits and the like to substrates such as printed circuit boards, particularly for ball grid arrays and similar technologies, is associated with the use of materials, having different temperature coefficients of expansion, in the integrated circuits and the substrates. As those skilled in the art will appreciate, the different materials used in the manufacture and/or packaging of integrated circuits and the fabrication of printed circuit boards tend to have different temperature coefficients of expansion. For example, the epoxy or ceramic material of an integrated circuit package has a different coefficient of expansion from the phenolic or epoxy material of a printed circuit board.
Thus, when temperature changes occur, the integrated circuit and the printed circuit board do not tend to expand or contract at the same rate. Such different rates of contraction and expansion result in a dimensional mismatch which may introduce undesirable stress concentrations in permanent interconnections, such as those resulting from the use of soldered joints and the like. Such undesirable stress concentrations may result in the formation of cracks in the interconnections. These cracks may eventually lead to failure of the interconnect to provide desired conductivity or electrical connection, thereby potentially resulting in failure of the entire electrical subassembly.
The effects of such temperature coefficient of expansion mismatches are particularly important in light of the fact that temperature changes are common in many electrical assemblies. Temperature changes typically occur in such electrical assemblies during power-up and power-down of the electrical assembly, as well as during normal environmental temperature changes. It should be appreciated that heat from the power supply, nearby electrical components and the integrated circuit itself may contribute substantially to such temperature changes. In view of the foregoing, it is desirable to provide techniques for mitigating the undesirable consequences of such mismatches of the temperature coefficient of expansion between the integrated circuits and printed circuit board.
Another problem typically associated with the attachment of integrated circuits and the like to substrates, such as printed wiring boards is that of poor electrical connection due to manufacturing tolerances which permit some of the individual connections to be inadequately conductive. Such inadequate conductivity results when one or more of the contacts of either the integrated circuit package or the printed wiring board is not flush or coplanar with the other contacts, such that the non-coplanar contact does not extend sufficiently far from the integrated circuit or printed wiring board to facilitate proper mechanical connection with its mating contact.
As used herein, the term Aintegrated circuit package@ is defined to include any device having electrical contacts formed thereupon for electrically interconnecting the integrated circuit die to a substrate. Such integrated circuit packages include chip-scale packaging (CSP), land grid array (LGA) packaging and ball grid array (BGA) packaging.
It is necessary for such mating contacts to be urged together with a sufficient amount of force to provide good mechanical interconnection thereof, so as to assure adequate electrical conductivity therebetween. In many instances, it is also necessary that sufficient force be provided so as to cause one of the contacts to penetrate an oxide layer of another of the contacts. In any instance, sufficient force is necessary so as to cause the contacts to abut over sufficient surface area at the mating interface thereof to provide the desired electrical conductivity therebetween.
Inadequate electrical connection of the mating contacts of an integrated circuit and a printed wiring board result in signal degradation, which may render the assembly inoperative.
While it is possible to improve the manufacturing tolerances of such devices as integrated circuits and printed wiring boards so as to mitigate the problems associated with inadequate electrical conduction, it is generally not desirable to do so because of the costs associated therewith. As those skilled in the art will appreciate, improving the tolerances of such devices so as to cause the electrical contacts thereof to be more nearly coplanar with one another involves substantial further processing and/or quality control. Such manufacturing procedures may, indeed, be cost prohibitive.
Interposers are frequently used in an attempt to mitigate the problems caused by inadequate electrical conductivity between the contacts of such devices as integrated circuits and printed wiring boards. Interposers typically comprise generally planar substrates having electrical contacts on each side thereof and having an electrical conduit between corresponding pairs of electrical contacts, so as to facilitate electrical communication therebetween. The planar substrate is formed so as to be flexible and the contacts are formed so as to be resilient or springy. The flexibility of the substrate compensates for differences in the height of the electrical contacts of the integrated circuit package and/or the printed wiring board. The resiliency of the contacts compensates for differences in the distance between complimentary pairs of contacts upon integrated circuit and packages and printed wiring boards and also assure adequate spring biasing force between the contacts of the interposer and the contacts of the integrated circuit package and/or printed wiring board. Interposers thus facilitate the use of devices such as integrated circuits and/or printed wiring boards having poor manufacturing tolerances while assuring adequate electrical connections therewith.
However, although such contemporary metal laden elastomeric interposers have proven generally suitable for their intended purpose, contemporary interposers do suffer from substantial deficiencies. Contemporary interposers generally comprise contacts having an elastomeric member which provides the resiliency or conforming property thereof, so as to allow the contacts of the interposer to compensate for differences in the height of the contacts of the integrated circuit package and/or printed wiring board.
When such an elastomeric member is exposed to substantial compression over an extended period of time, such as when the interposer is in use, then the elastomeric properties thereof may tend to degrade in a manner which substantially mitigates the resiliency thereof. That is, over time the elastomer may tend to break down and lose at least a portion of its spring force, such that the elastomer no longer urges the contacts of the interposer toward the contacts of the integrated circuit package and/or the contacts of the printed wiring board with sufficient force to assure adequate electrical conductivity therebetween.
As such, it is sometimes desirable to avoid the use of such elastomers in the construction of interposers, at least insofar as the elastomer is used to effect spring biasing of the electrical contacts of the interposer toward the electrical contacts of the integrated circuit package and/or the electrical contacts of the printed wiring board.
Further, conductive elastomers tend to have an undesirably higher contact resistance than direct metallic contacts. The bulk resistance of conductive elastomers is also undesirably higher than that of corresponding metal contacts. The contact resistance and bulk resistance of conductive elastomers places a constraint upon the smallest pitch size which is acceptable in an array of such contacts. That is, the pitch size must be sufficient to facilitate the fabrication of conductive contacts having a large enough size so as to provide an acceptable contact resistance and bulk resistance.
Metal springs do not tend to degrade substantially when compressed over extended periods of time. Further, metal springs used as electrical contacts do not have an undesirably high contact resistance and/or an undesirably high bulk resistance, and therefore do not impose the above undesirable constraints upon the size to which the pitch of such contacts may be reduced in an array. However, it is difficult to manufacture metal springs which are sufficiently small as to be capable of providing the desired electrical contact between electrical devices such as an integrated circuit and a printed wiring board and which are also capable of providing sufficient force so as to assure adequate electrical conductivity therebetween.
Because of the high density of electrical contacts on the package of contemporary integrated circuits, it is necessary that each electrical contact be very small. Many contemporary integrated circuits have between approximately 200 and approximately 2,000 input/output contacts formed thereon. These electrical contacts are typically formed in a pattern having a distance of 1.27 mm therebetween, center-to-center. Emerging LGA and BGA technologies utilize electrical contacts formed in a pattern having a distance of 1 mm therebetween, center-to-center. Minimalist packages utilizing electrical contacts formed in a pattern having distances of 0.5 mm to 0.8 mm, center-to-center, are commonly used in such applications as Rambus Dynamic Random Access Memory (RDRAM) devices.
Because of their small size, it is extremely difficult to form metallic springs having sufficient spring force to effect desired electrical conductivity. It is also difficult to form very small arrays of metallic springs which are positioned or juxtaposed sufficiently close to one another to serve as electrical contacts for the interconnection of integrated circuits and the like. For example, it is possible to construct very small springs utilizing hardened phosphor bronze or beryllium copper. However, the phosphor bronze or beryllium copper must be hardened after it has been formed into the desired spring shape. Contemporary microstructure spring forming processes which include techniques such as photolithography and electrodeposition require that the spring be formed upon the substrate of an interposer or the like. Such interposer substrates comprise a polymer material, such as a polyimide film or a polyester film. One example of such a film is KAPTON (a registered trademark of E.I. du Pont de Nemours and Company of Circleville, Ohio).
Thus, such contemporary methodologies necessitate that phosphor bronze or beryllium copper springs be hardened in situ, upon the polymer substrate. However, as those skilled in the art will appreciate, the temperatures to which the phosphor bronze or beryllium copper must be raised in order to effect hardening are not compatible with the polymer substrate. Such elevated temperatures cause degradation of the polymer substrate which renders it incapable of functioning in its intended use.
In view of the foregoing, it is desirable to provide an interposer having metallic springs which are capable of being manufactured utilizing contemporary manufacturing techniques and which are capable of providing adequate electrical conductivity of electrical connections made therewith.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a microstructure spring formed by applying a photoresist to a substrate; aligning a mask with the substrate, the mask defining a pattern representative of a spring; exposing the substrate to electromagnetic radiation, such as ultraviolet light, so as to polymerize the photoresist, thereby allowing the masked areas to develop away and removing an undeveloped portion of the photoresist. Alternating layers of copper and nickel are then formed upon the substrate, such as by electrodeposition. The developed photoresist is then removed and the alternating layers of copper and nickel formed upon the substrate where the photoresist was absent define the spring.
As those skilled in the art will appreciate, this process facilitates the fabrication of microcomposite springs which are suitable for use in applications such as the spring biasing of electrical contacts of an interposer which is used to attach electrical devices such as the packages of integrated circuits and printed wiring boards to one another.
These, as well as other advantages of the present invention will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.